An array of specific serotypes have been identified within the species Paramecium tetraurelia on the basis of specific responses to diverse immune sera. This response is the result of a surface localized antigen (i-antigen) which covers the entire cell except for a region along the oral suture (midline). Numerous studies hae been carried out on the surface i-antigens, but these studies have been restricted in every case to those of a single species - P. tetraaurelia. This project involves the identification, purification and characterization of the surface antigens from another species, P. multimicronucleatum.